It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
The modern technological era relies on a steady, reliable supply of energy, for use in all aspects of everyday life. One of the most attractive ways to store and transport energy produced from renewable sources is based on splitting of water into oxygen and hydrogen.
Water splitting is the separation of water into its constituents—oxygen (O2) and hydrogen (H2). Photo-electrochemical water splitting involves breaking down water into hydrogen and oxygen by electrolysis, but the electrical energy is supplied from a photo-electrochemical cell (PEC) process. This system is often colloquially referred to as ‘artificial photosynthesis’.
Water splitting is one of the simplest ways to produce high purity hydrogen. Although the efficiency of water electrolysis lies in the range of 50-70%, the cost of hydrogen produced by this method is in the range of $20-30/GJ (assuming $0.05/kWh), compared to $6-12/GJ produced via natural gas reforming and coal gasification. (S A Sherif, F Barbir and T N Veziroglu, Solar Energy 2005, 78, 647-660)
If the water splitting process is assisted by photo-catalysts suspended directly in water instead of using photovoltaic and an electrolytic system the reaction comprises a single step and is therefore more efficient.
Photo-catalysis or photo-electrochemical splitting of water into hydrogen and oxygen can be used to collect and store energy on a global scale. In essence it consists of capturing energy and storing it in the form of chemical bonds to yield solar fuels which can be used as needed. Thus water splitting to produce hydrogen is a potentially major post-petroleum energy solution.
For example, in one future scenario rooftop solar panels could provide electricity to a home, and any excess electricity would be directed to an electrolyser (that is, a device for splitting water molecules) to produce hydrogen, which would be stored in tanks. When more energy was needed, the hydrogen would be fed to a fuel cell, where it would combine with oxygen from the air to form water, and generate electricity at the same time.
An electrolyser comprises two different electrodes, one which releases oxygen molecules and the other which releases hydrogen molecules. Although the half-reaction that produces hydrogen provides the storable source of energy, the half-reaction that produces oxygen is more difficult to optimise and control. Specifically, the half-reaction to produce oxygen is particularly demanding because it requires the distribution of four redox processes over a narrow potential range, the coupling of multiple proton and electron transfers, and the formation of two oxygen-oxygen bonds. Controlling parameters to optimise the efficiency and the conditions under which the reaction occurs is key to the overall viability of energy storage via water splitting.
However one major problem in obtaining efficient water splitting devices has been the stability of, and the significant over-potential on the oxygen producing electrode.
Overpotential is a term well known to those skilled in the electrochemical art and refers to the potential (voltage) difference between a half-reaction's thermodynamically determined reduction potential and the experimentally observed potential at which the redox event occurs. Overpotential is directly related to a cell's voltage efficiency. In an electrolytic cell the overpotential requires more energy than thermodynamically expected to drive a reaction and the energy difference is lost as heat. Overpotential is specific to each cell design and can vary between cells and operational conditions even for the same reaction.
One of the main obstacles to improved efficiency of the water splitting processes lies in development of catalysts that meet the broad requirements of practical applications. Ideally the catalyst should be based on abundant, low cost materials, have high turnover frequencies, remain active over prolong periods of time and be able to regenerate itself.
The oxygen producing electrode is commonly made of materials such as platinum, manganese oxides and Mischmetal oxides. Mischmetals are typically alloys of rare earth elements in various naturally-occurring proportions, and include cerium mischmetal, rare earth mischmetal or just ‘misch metal’. A typical composition includes approximately 50% cerium and 25% lanthanum, with small amounts of neodymium and praseodymium. Scarce metal elements such as iridium and cobalt have also been included in the composition of the electrode but add significantly to the cost of the electrode.
The broad range of water oxidation catalysts developed so far allows for water oxidation in concentrated basic solutions (pH>13) by materials based on the perovskite metal oxides (RuO2, IrO2, Co3O4, MnO2 etc.) and under neutral or acidic conditions (pH<1) by precious metals and their oxides (e.g. Pt, PtO2). The over-potentials achieved by noble metal catalysts are around 320 mV.
Among heterogeneous oxygen-evolving catalysts, one of the highest activities was produced from material formed upon anodic polarization in phosphate solutions containing Co(II). Although the catalyst is able to oxidize water in neutral pH and at room temperature water oxidation occurred at potential around 1.2V vs NHE which corresponds to about 400 mV of over-potential, indicating that improvements are needed before this catalyst will be energetically efficient.
Furthermore, in recent times novel, improved oxygen producing electrodes have been made from cobalt phosphates and iridium oxides and reportedly have over-potentials of 0.4V and 0.25V respectively. (Y. Surendranath, M. W. Kanan, D. G. Nocera, J. Am. Chem. Soc. 2010, 132, 16501-16509; S. D. Tilley, M. Cornuz, K. Sivula, M. Graetzel, Angew. Chem. Int. Ed. 2010, 49, 6405-6408). Despite the improved performance of these novel materials as compared with the prior art, the over-potential is still substantial and they are comparatively expensive due to the inclusion of the scarce metal elements.
Thus there has been a continuing search to satisfy the need for better and more cost efficient materials for use as catalysts. There is also a need to improve the efficiency and stability of catalytic reactions, including photo-electrochemical reactions such as water splitting.
Consistent with this, there is a need for improved methods of creating catalysts.